The most important biological role yet recognized for the metal manganese (Mn) is its involvement within the Water Oxidation Center that catalyzes the conversion of water to oxygen gas within the photosynthetic apparatus of green plants and cyanobacteria. This reaction represents the source of atmospheric oxygen for supporting respiring life. The WOC consists of an oxide-bridged Mn4 aggregate of unknown structure at which the water-to-oxygen transformation is accomplished by a currently unknown mechanism. The objective of this program is to contribute to the elucidation of the structural identity and mechanism of action of this system. Believing that the Mn4 aggregate is a thermodynamically-stable entity capable of independent existence outside the polypeptide environment in which it naturally resides, this program is seeking the synthesis of this aggregate using inorganic synthesis procedures and simple Mn reagents. Synthetic access into the appropriate type of cluster chemistry has already been achieved and initial products are being studied with a greater number of techniques, and data are being obtained to a higher level of precision, than is possible for the native WOC Mn site. The major objective is the study of stable species at all the various oxidation levels adopted by the native WOC site during the catalytic cycle to allow assessment of the changes in structure and properties that occur. The ultimate objective is to accomplish the native substrate transformation viz water oxidation to O2 gas. Reproduction of the biological reaction under controlled laboratory conditions using synthetic aggregates allows the possibility of obtaining mechanistic information and detecting transient intermediates for the final and all-important step of elimination of oxygen gas.